The present invention relates to a duplex stainless steel with high contents of Cr, Mo and N. The content of ferrite is 30-70%. The material is especially suited for production tubes for extraction of crude oil and gas, but can also be used in applications where good corrosion resistance together with high strength is required.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Duplex steels are characterized by an austenite-ferrite structure where both phases have different chemical compositions. Modern duplex stainless steels will mainly be alloyed with Cr, Mo, Y and N. Swedish Patent 8504131-7 describes a duplex stainless steel grade with commercial denotation SAF 2507 (UNS S32750), which is mainly alloyed with high contents of Cr, Mo and N for good resistance to pitting corrosion. This resistance is often described with a PRE-number (PRE=Pitting Resistance Equivalent=% Cr+3.3% Mo+16% N). Thus, the alloy is consequently optimized with respect to this property, and has certainly good resistance in many acids and bases, but above all the alloy is developed for resistance against chloride environments. Cu and W were subsequently also used as alloying additions. Consequently, a steel grade with commercial denotation DP3W has a composition similar in character as SAF 2507, but it has been alloyed with 2.0% W as substitute for a part of the Mo content in the alloy. A steel grade with commercial denotation Zeron 100 is a further steel grade of a similar kind as SAF 2507, but this is alloyed with approximately 0.7% Cu and approximately 0.7% W. All above-described steel grades have a PRE-number higher than 40 irrespective to the method of calculation.
Another type of duplex alloy with high resistance to chloride is the steel grade described in the Swedish Patent 9302139-2. This alloy is characterized by Mn 0.3-4%, Cr 28-35%, Ni 3-10%, Mo 1-3%, Cu max 1.0% and W max 2.0%, and has a high PRE-number above 40. The biggest difference compared to the established superduplex steels SAF 2507 and others is that the contents of Cr and N are higher in this steel grade. The steel grade has found use in environments where resistance to intergranular corrosion and corrosion in ammonium carbamate is of importance, but the alloy has also a very high resistance to corrosion in chloride environments.
In oil and gas extraction applications, duplex steels are used in the form of production tubes, e.g.xe2x80x94tubes that transport oil up from the source to the oil-rig. Oil wells contain carbon dioxide (CO2) and sometimes even hydrogen sulphide (H2S). An oil well containing CO2 but no bigger multitudes of H2S is called a sweet oil well. A sour oil well, however, contains H2S in varying amounts.
The production tubes will be supplied in threaded finish. By means of couplings the tubes will be joined to the necessary lengths. Because oil wells are situated at a considerable depth the length of a production tube can become large. Demands on the material, which shall be used in this application, can be summarized according to the following:
* Yield point in tension min 110 ksi (760 MPA)
* Resistance to corrosion caused by CO2 or H2S. Material should be qualified and included in for example the standard NACE MR-0175
* Good impact strength down to xe2x88x9246xc2x0 C., at least 50J
* Further the material shall be possible to manufacture in the shape of seamless tubes as well as that one can produce threads and fitting couplings for tubes.
In the present-day situation either low alloyed carbon steels, austenitic stainless steel, duplex stainless steel or nickel-based alloys, are used for such applications, depending on the level of corrosive activity in the oil well. Limits for different materials have been taken out. For sweet oil wells one can normally use carbon-steel or low alloyed stainless steel, for example, martensitic 13Cr-steel. In sour oil wells, where the partial pressure for H2S exceeds 0.01 psi, normally the use of a stainless steel is required.
Duplex steels are, among other things, are an economical alternative to stainless steels and nickel-based alloys, thanks to a low content of nickel. The duplex steels fill the gap between high-alloyed steels and low-alloyed carbon steels and martensitic 13Cr-steel. A typical application range for duplex steels of the type 22Cr and 25Cr is where the partial pressure of H2S in the gas in the oil well lies in the area 0.2 to 5 psi.
Since there is a requirement on the strength level of at least 110 ksi, 22Cr-och 25Cr-steel is supplied with a cold rolled finish, which increases the strength to desired level, but this also limits the resistance of the material against stress corrosion caused by H2S. Material of the type 22Cr, in an annealed condition, has only a yield point limit of 75 ksi, a corresponding value for 25Cr is 80 ksi. Besides, from the production point of view it is difficult to produce production tubes from such materials, because the strength depends of both the total degree of reduction and the type of method for the reduction, i.e.xe2x80x94drawing or rolling. Additionally, a cold rolling operation is costly for the production. The impact toughness of the material deteriorates considerably by the cold rolling, which further limits the applicability of such materials.
In order to solve these problems there is a need of an alloy which can be delivered in a hot extruded and annealed finish, where the strength is at least 110 ksi. Simultaneously, the alloy shall have good workability and, without problems, can be extruded into seamless tubes. The strength of duplex alloys can be increased by alloying with high contents of the elements Cr, Mo and N. In the present day situation there are duplex steels with up to 29% Cr and 0.4% N, which have yield point limits of 95 ksi, but in this alloy the content of Mo must be held low in order to avoid precipitations of, for example, the phase. When the content of Mo is high, the content of Cr has to be reduced to approximately 25% if one wants to retain the structural stability. Thus, there seems to exist an upper limit for the combination of Cr and Mo in order to retain the structural stability. The content of N is limited upwards to 0.3%, for 25% Cr alloys and to 0.4% for 29% Cr alloys.